Conventionally, there has been known a piston pump which rotates a cylinder block apparatus via a drive shaft by a power of a drive source and oscillates a piston within a cylinder block, thereby sucking an oil from a tank and discharging a high pressurized oil so as to convert a mechanical energy to a fluid energy. Further, there has been known a piston motor which introduces a high pressurized oil within a cylinder block from a pump and oscillates a piston, thereby rotating a cylinder block apparatus and a drive shaft so as to convert a fluid energy to a mechanical energy. In this case, a basic structure of the cylinder block apparatus is common in both of the piston pump and the piston motor.
An embodiment of an inclined shaft type piston motor having a cylinder block apparatus 30 will be shown in FIG. 6. A drive shaft 31 is supported by bearings 33 and 34 received in a case 32 so as to be rotated. A flange portion 31a is integrally formed in an end side of the drive shaft 31. A ball 35a integrally formed with a center shaft 35 is assembled on a rotary shaft core X of the drive shaft 31 in the flange portion 31a, and the center shaft 35 is oscillated in a vertical direction at a predetermined inclination angle with respect to the rotary shaft core X of the drive shaft 31 an inclination angle control apparatus 60.
A plurality of piston assemblies 36 are arranged in the flange portion 31a from the rotary shaft core X of the drive shaft 31 in such a manner as to be on the same circumference. The piston assemblies 36 are constituted by piston rods 37 and pistons 38 and slidably connected. The piston rod 37 has spherical portions 37a and 37b at both ends and both portions are connected by a rod 37c.
The piston 38 is constituted by a circular column having a circular hole pierced in an axial direction from a side of an end surface, and a bottom of the hole is formed in a semispherical shape. The spherical portion 37a at one end portion of the piston rod 37 is inserted to the semispherical portion in the bottom of the hole of the piston 38, and both elements are connected by deforming an outer diameter of the piston 38. The piston 38 can be oscillated in a range at which the piston rod 37 is brought into contact with the hole. Further, the spherical portion 37b in the other end portion of the piston rod 37 is mounted to the flange portion 31a of the drive shaft 31 in such a manner as to freely oscillate. Accordingly, the piston 38 is mounted to each of the drive shaft 31 and the piston rod 37 in such a manner as to freely oscillate. An outer diameter in a side of the other end surface of the piston 38 is inserted to a cylinder block 42 mentioned below in a sealed manner, thereby sealing a high pressurized oil acting on the side of the other end surface of the piston 38 by an outer circumference portion 38a (shown in FIGS. 7A and 7B) of the piston 38.
A shape of the outer circumference portion 38a of the piston 38 includes a straight shape without a groove (FIG. 7A) and a shape in which a plurality of labyrinth grooves 38b not communicating with each other in a longitudinal direction are cut (FIG. 7B).
A case drain 39 shown in FIG. 6 is formed in an inner portion of the case 32, and an oil leaking from a gap between the piston assembly 36 and the cylinder block 42 is discharged from a drain port 41 to a tank (not shown) via a case drain 39. Since a pressure is uniformly distributed all around the periphery due to a function of the labyrinth groove 38b, the piston 38 is held near a center of the hole 42b of the cylinder block 42. As a result, the piston 38 is not directly brought into contact with the hole 42b even when the piston 38 oscillates within the hole 42b, so that a heat generation due to a sliding friction can be restricted to a low level. Further, since the labyrinth groove 38b projects to a side of the case drain 39 having a low temperature from the cylinder block 42 due to an oscillation of the piston 38, the high temperature oil in the labyrinth groove 38b can be discharged or cooled.
The cylindrical cylinder block 42 shown in FIG. 8 oscillates in a vertical direction with respect to the rotary shaft core X of the drive shaft 31 in accordance with an oscillation in a vertical direction of a center shaft 35 by the inclination angle control apparatus 60 mentioned above. Accordingly, the cylinder block 42 rotates around a rotary shaft core Y of the center shaft 35.
The side of one end surface of the cylinder block 42 is formed in a concave spherical surface shape, and the spherical surface has a plurality of suction and discharge ports 42a and is slidably brought into contact with a convex spherical surface of the valve plate 43. A plurality of cylinder block holes 42b (hereinafter, refer to as cylinder holes 42b) are pierced in the side of the other end surface of the cylinder block 42 at the same number as that of the piston assemblies 36 mounted to the flange portion 31a at an equal interval on a circumference inside the cylinder block 42. These cylinder holes 42b are connected to a plurality of suction and discharge ports 42a, and a plurality of piston assemblies 36 are inserted to each of the cylinder holes 42b at a sealing interval in such a manner as to freely oscillate. The high pressurized oil from each of the suction and discharge ports 42a acts on the end surface of each of the piston assemblies 36.
The ball 35a in the side of one end of the center shaft 35 is assembled in the flange portion 31a, however, the side of the other end is supported by the bearing 44 of the valve plate 43. The valve plate inclines on a sliding surface 45 having a concave spherical surface shape and formed in the inclination angle control apparatus 60 around a core Z of the ball 35a of the center shaft 35. In this case, the inclination angle corresponds to an inclination of the rotary core Y of the cylinder block 42 with respect to the rotary shaft core X of the drive shaft 31, and is adjusted by the inclination angle control apparatus 60.
In this case, when adjusting the inclination angle a little, the cylinder block 42 comes near to the rotary shaft core X of the drive shaft 31, so that the piston assembly 36 is further inserted within the cylinder block 42 and a stroke S (a difference of at amount between forward and backward positions of the piston) becomes small. As a result, since a space capacity between the cylinder block 42 and the piston assembly 36 is reduced, a number of oscillation per a unit time of the piston assembly 36 is increased in the case of the constant inlet amount, so that a number of rotation of the drive shaft 31 connected to the cylinder block 42 is increased. That is, when the inclination angle is reduced, it becomes a high speed rotation, and inversely when the inclination angle is increased, it becomes a low speed rotation. Further, when the inclination angle is 0, that is, the rotary shaft core X of the drive shaft 31 and the rotary shaft core Y of the cylinder block 42 are on the same axis, the stroke S becomes 0, the piston assembly 36 is not going to oscillate, and the drive shaft 31 is not going to rotate.
A sheet 46 and a spring 47 are arranged between the center shaft 35 and the cylinder block 42, thereby keeping a contact state in the spherical sliding surface formed by the cylinder block 42 and the valve plate 43 by a pressing force of the spring 47. The suction and discharge port 42a of the cylinder block 42 is connected to an inlet for a pressurized and a discharge oil outlet (not shown) of the valve plate 43.
In accordance with the structure mentioned above, since the cylinder block apparatus 30 cools and lubricates by the high pressurized oil leaked from the gap with respect to the cylinder block 42 or the oil stored in the labyrinth groove 38b on the outer periphery of the piston 38 even when the piston assembly 36 oscillates, it is possible to prevent a sliding friction heat and a seizure generated by an oscillation of the piston assembly 36 within the cylinder block 42.
However, a market of the piston pump and motor tends to a high speed (a high rotational speed) in view of an embodiment of a high speed travel of the hydraulic excavator. When the rotation is performed at a high speed, the upper piston assembly 36 in FIG. 6 is further inserted within the cylinder block 42, an insertion depth of the lower piston assembly 36 becomes small, and the stroke S becomes small. Since the number of oscillation per a unit time becomes more in accordance with this, the opposing circumferential surfaces of the piston assembly 36 and the cylinder block 42 are locally heated to be a high temperature. Further, when the piston assembly 36 is inserted within the cylinder block 42 and the stroke S becomes small, the labyrinth groove 38b hardly protrudes to the side of the low temperature case drain 38 from the cylinder block 42, so that the high temperature oil stored in the labyrinth groove 38b is not discharged, and a cooling effect is reduced.
Further, when the cylinder block apparatus 30 rotates at a high speed around the rotary shaft core Y of the cylinder block 42, the piston assembly 36 is shifted to the side of the outer periphery of the cylinder hole 42b in the cylinder block 42 due to a centrifugal force. As a result, as shown in FIG. 5, since the piston assembly 36 is exposed to overlapped bad conditions that an oscillating motion at a high cycle is performed under a state of being strongly pressed to the outer peripheral surface of the hole 42b, a heat generation due to the sliding friction and the seizure are locally generated. This phenomenon becomes significant as the piston pump and motor are made high speed (high rotational speed).
Further, there is a method of increasing a gap between the cylinder block 42 and the piston 38 in order to prevent the heat generation due to the sliding friction mentioned above and increasing an amount of the oil leaking to the case drain 39 so as to cool. However, a capacity efficiency (a ratio between an actual discharge amount and a theoretical discharge amount including a leakage and the like in the case of the pump, and a ratio between a theoretical inlet amount and an actual inlet amount in the case of the motor) corresponding to a basic performance of the piston pump and motor is reduced. In particular, in an area of the number of rotational of about 500 rpm at the low speed, it is significantly reduced.
FIG. 9 is a graph obtained by actually measuring a relation of a gap of a diameter (.mu.m), a leaking amount (l/min) and a capacity efficiency (%) between each of the piston and the cylinder hole 42b of the piston and the cylinder block 42 with respect to the piston motor having a rated capacity (160 cc/rev) with changing a pressure. That is, the leaking amount and the capacity efficiency at symbols B and C are shown in Table 1 with setting the gap of the diameter of a symbol A to a reference. A piston in which a plurality of labyrinth grooves 38b shown in FIG. 7B are cut is employed as the piston 38.
TABLE 1 EXPERIMENTAL VALUE OF GAP OF DIAMETER. LEAKING AMOUNT AND CAPACITY EFFICIENCY GAP OF LEAKING CAPACITY DIAMETER AMOUNT EFFICIENCY SYMBOL (pm) (1/min) (%) A REFERENCE REFERENCE REFERENCE B REFERENCE + 10 REFERENCE + 3 REFERENCE - 4 C REFERENCE + 20 REFERENCE + 7 REFERENCE - 9 SET GAP OF A DIAMETER TO REFERENCE PRESSURE: 350 kg/cm.sup.2 NUMBER OF ROTATION: 500 rpm
In Table 1, in the case of the symbol B, when making the gap of the diameter between the piston 38 and the cylinder hole 42b of the cylinder block 412 10 .mu.m greater than the reference gap of the diameter at the symbol A, the leaking amount is 3 l/min increased and the capacity efficiency is 4% reduced. In the case of the symbol C in which the gap of the diameter is made 20 .mu.m greater, the leaking amount is 7 l/min increased and the capacity efficiency is 9% reduced. As mentioned above, since the capacity efficiency is reduced, thereby generating a heat when the gap of the diameter is made greater, a method of increasing the gap can not be employed.